In an exhaust gas discharged from a diesel vehicle is contained nitrogen oxide (NOx) in addition to carbon monoxide (CO) and hydrocarbon (HC). Thus, in recent years, it has been practiced to reduce the toxic nitrogen oxide into a nontoxic gas. For example, it has been proposed to dispose a NOx selective reduction (SCR) catalyst in an exhaust gas discharge muffler of a diesel vehicle, put urea water serving as a reducing agent solution into a tank separately provided to the vehicle and spray the urea water over the above-described catalyst for thereby reducing NOx into nontoxic gas such as N2. Since this system becomes incapable of urging NOx reduction but causes a large amount of discharge of NOx if urea water runs out, it has been taken such a measure of providing a container (hereinafter, also called a tank) for containing urea water with a sensor for measuring a liquid level (hereinafter, also called a water level) of urea water to be contained and issue a warning if the remaining amount of urea water becomes equal to or lower than a predetermined amount.
As an example of a sensor for measuring the water lever, there is known an electrostatic capacity type liquid state detecting sensor. The liquid state detecting sensor includes an elongated tube made of a conductor and serving as an outer electrode (outer tubular electrode) and measures an electrostatic capacity between the outer tubular electrode and an inner electrode in the form of an elongated post or pipe and disposed along the axial direction of and concentrically within the outer tubular electrode for detecting the water level from the electrostatic capacity. For example, in an electrostatic capacity type liquid state detecting sensor used for measuring a water level of liquid having a conductivity such as urea water, an insulation layer for preventing a short between the outer tubular electrode and the inner electrode is formed on the surface of the inner electrode and moreover a liquid state detecting sensor is set at a tank that serves as an object to be measured in such a manner that the axial direction of the outer tubular electrode coincides with the up and down direction of the water level. In case of measurement of a water level of such electrically conductive liquid, the electrostatic capacity of a portion that is not immersed in liquid depends upon an air layer in a gap between the inner and outer electrodes and the thickness of the insulation layer of the inner electrode. On the other hand, the electrostatic capacity of a portion immersed in the liquid depends only upon the thickness of the insulation layer since the conductive liquid becomes equal to the outer tubular electrode in the potential and becomes larger in the electrostatic capacity than the former. For this reason, the measured electrostatic capacity becomes larger as the portion immersed in the liquid increases, thus making it possible to detect the water level.
It is usual that such a liquid state detecting sensor is installed inside the tank in such a manner that the axial direction of the outer tubular electrode coincides with the up and down direction of the water level. For example, in case the outer tubular electrode (and inner electrode) is installed in a state of being suspended from a ceiling of the tank to the inside thereof, the liquid state detecting sensor is fixed (or supported) at the base (upper) end of the outer tubular electrode to (or on) a base end support member having a means for installation on the tank so that the base end side of the outer tubular electrode is disposed on the ceiling side. On the other hand, the inner electrode is fixed at a base end portion to the base end side of the outer tubular electrode in a way as to be disposed inside the outer tubular electrode and maintain insulation between the inner electrode and the outer tubular electrode. However, since the respective electrodes are put into, so to speak, a cantilever support condition of being free at the leading end sides by such fixing and supporting of the both electrodes only at the base end portions thereof, inclination of the respective electrodes in the radial direction or bending deformation thereof may occur when the sensor is put in such a using condition where the sensor is subjected to vibrations or in a condition where an external force is applied to the sensor along the lateral direction thereof. For this reason, the dimensions of the space between the both electrodes become unstable or contact of the both electrodes may be caused in some case, thus making it impossible to measure the electrostatic capacity accurately. Further, by repetition of such inclination or deformation, a large stress may be caused in each electrode, particularly at the base of the inner electrode, and therefore there is a danger of the electrode being broken.
For this reason, it has been practiced, in such a sensor, to dispose a spacer or a support member between the both electrodes, which serves as a means for stabilizing the dimensions of the space between the electrodes or an inner electrode support means for holding the dimensions of the space between the both electrodes constant. For example, a technique of disposing a plurality of spacers made of an insulating material around and longitudinally of the electrode so that the both inner and outer electrodes are held concentric is known (Patent Document 1). Further, a technique of disposing, at the leading end portions of the both inner and outer electrodes, a support member made of insulating resin between the electrodes and holding the interval (dimensions of the space) between the both electrodes constant while insulating between the electrodes by the support member (Patent Document 2).
Patent Document 1: Published Japanese Utility Model Application No. 1-151215.
Patent Document 2: Published Japanese Patent Application No. 9-152368
However, since the support means for the inner electrode in the former document requires a plurality of spacers, increase in the number of parts is incurred. Further, since it is necessitated to first dispose the spacers on the outer circumferential surface of the inner electrode and thereafter insert them under pressure (insert them) into the outer electrode through one end opening, the assembly work is difficult. Further, the technique of disposing the support member made of resin between the electrodes is encountered by the following problem since the support member is made of resin and therefore it is difficult for vibrations and external force to be absorbed by the support member between the both electrodes. First, when the support member is disposed between the both electrodes (leading ends) under a condition where the axes of the both electrodes are not correctly in line due to some error in manufacture and assembly, a lateral residual stress is caused in each electrode or each electrode is acted upon by a partial load and therefore there may be caused such a case in which one of the electrodes is broken at the base thereof. On the other hand, to prevent occurrence of such a case, it is necessitated to make considerably higher the dimensional accuracy of the parts including the electrodes, and therefore increase in the manufacturing cost is incurred. Further, since the usual support member made of resin is hard, vibrations are transmitted between the both electrodes by way of the support member. Thus, in case of, for example, such an inner electrode that has an insulating layer on the surface, there is a danger of the insulating layer being liable to be damaged and the insulating ability being destroyed.